I'm having the same issue, only a few months later.
I was informed that an update was available (which routinely happens mid month) and without too much concern began the update process.
The machine presumably had to reboot to complete the install, and after about 20 min I was at a blue screen with the notice that it was updating. 4+ hours later, the screen hadn't changed. It would be nice if MS had bothered to put some sort of percent update status, but nothing except an endlessly spiraling circle.
I assumed that the install had stalled, which has happened in the past (gee, thanks Microsoft), so I did a hard reboot. That resulted in a black screen with the line "undoing changes." 3+ hours later, it finally came up to the desktop, but everything was glacially slow. I clicked on settings (waited 10+ min), clicked on updates (waited 10+ min) and saw an update status of "installing - 23%."
I though this a bit reassuring, so I went to bed, certain that by morning all would be well.
Morning.
Things are kinda working -- my off-line backup apparently worked without issue -- but that 23% had only crept up to 52% in 6 hours. Now, close to 20 hours in, and it's only at 75%.
Over the decades, I've fought through myriad updates. I was there in the DOS days and when Windows 3 (3.1) became Windows 95. But this latest has been the worst.
Who needs a computer virus when you have Windows?
LOL! "Who needs a virus...", that's the best one yet.
I am hearing this stuff, and it's chilling. It makes no sense that there are so many people with this 'slow update install' issue. Then there are others, like me, who've been getting them done in minutes. I have had only two customers, so far, that have had perpetual installs that had to be stopped. But I was able to get them done in, the longest, two hours. I just don't get it. And I'm not getting much from the Insider support group that is helpful. It seems too many wanna blame the system. I'm not buying that, unless, of course, MS is going into the hardware business!!!